Conrad Harrison wrote, “They are not supposed to be made from hay, just straw. Straw is mostly cellulose fibre, and apparently somewhat termite resistant. Hay as different characteristics and is undesirable in the bales.”
I hope I’m not stating something everyone knows. Hay made from grass is cut green and includes the developing seed head. After grass has made its seeds and dried while standing in the field is cut and the seeds threshed out, the stems that are left is straw. It is gold in color, which is why Rumplestiltzkin could have his prisoner spin straw into gold. Properly stored hay retains a green hue and contains many more organics, which decay more easily than straw, undesirable in a structure.